1. Field of the Invention
The present invention relates to a compact helical antenna for use in a mobile communication terminal, a local area network (LAN), or the like, and also relates to a communication apparatus incorporating the same.
2. Description of the Related Art
FIG. 18 is a perspective view showing one example of an antenna and its mounting method employed in a conventional mobile communication terminal. As seen from the figure, in general, a whip antenna 21 is mounted in a casing 22 of the mobile communication terminal.
In recent years, in keeping with advancement of mobile communication technology and diversification of customer services, portable terminals have been coming into wide use. In consideration of carryability, the communication terminals have come to have an increasingly smaller casing. With this trend, miniaturization and weight reduction have been underway in components which are incorporated or mounted in the communication terminals. Contrary to this current situation, the conventional whip antenna 21 is so configured as to protrude from the casing 22. In order to achieve further miniaturization of terminals, there is a demand for a downsized and lightweight antenna which is designed not to jut from a casing.
To satisfy such requirements, as a compact antenna, a helical antenna has been under development that has a radiating electrode composed of a conductor taking on a helical structure.
FIG. 19 is a perspective view showing a helical antenna disclosed in Japanese Unexamined Patent Publication JP-A 9-121113 (1997). This helical antenna is constructed by arranging a helically-configured conductor 15 within a base body 11. The conductor 15 is connected, at a feeding end 17, to a connecting portion of a terminal electrode 12 disposed at one end face of the base body 11, and is wound in a spiral fashion in the direction of the length of the base body 11. In this way, by forming the conductor 15, acting as a radiating electrode, in a helical shape, miniaturization of the antenna can be achieved.
In such a helical antenna, its resonant frequency is determined in accordance with the number of winding of the helically-configured conductor (=conductor length); the width of the conductor; the size (thickness, length, and width) of the base body; and the relative dielectric constant.
However, the following problem arises. The helical antenna, which is downsized by helically configuring the conductor, is susceptible particularly to an increase of capacity in conductor patterns and an influence of electrical connection. As a result, the resonant frequency tends to be greatly affected by a variation in the width of the conductor.
For example, depending on the method for constructing the conductor of such a downsized helical antenna, there may occur a nearly 5% dimensional variation in the conductor width. In this case, even if the helical antenna is so designed as to obtain a desired resonant frequency, it is inevitable that a great, nearly 5% variation in the resonant frequency occurs due to the variation of the conductor width caused in course of manufacture.
The invention has been devised in view of the above stated problems with the conventional art, and accordingly one object of the invention is to provide a compact helical antenna in which, even if there is for example a 5% variation in the conductor width, a variation in the resonant frequency can be reduced to 1% or below.
Another object of the invention is to provide a communication apparatus which is excellent in antenna characteristics stability, and incorporates a compact helical antenna in which, even if there is for example a 5% variation in the conductor width, a variation in the resonant frequency can be reduced to 1% or below.
Note that, if the frequency variation is reduced to 1% or below, the helical antenna, when used in PDC (Personal Digital Cellular), PHS (Personal Handyphone System), Bluetooth, or other systems, succeeds in satisfying specific frequency standards thereof.
As a result of conducting extensive research and study on the conductor patternxe2x80x94resonant frequency relationship as observed in a helical antenna, the inventors of the present application have found that the use of a helical antenna having a subsequently-described structure makes it possible to solve the above stated problems. Thereupon, the present invention is accomplished.
The invention provides a helical antenna comprising a base body made of a dielectric material or a magnetic material, and a helically-configured conductor formed at least either on a top surface of the base body or in an interior thereof,
wherein, in the base body, a thickness a (mm) is kept in a range of 0.3xe2x89xa6axe2x89xa63 (mm); a length b (mm) is kept in a range of 5xe2x89xa6bxe2x89xa620 (mm); and a relative dielectric constant xcex5r is kept in a range of 3xe2x89xa6xcex5rxe2x89xa630 or a relative magnetic permeability xcexcr is kept in a range of 1xe2x89xa6xcexcrxe2x89xa68, and also a number of winding x (turn) of the conductor is kept in a range of 3xe2x89xa6xxe2x89xa616,
and wherein a resonant frequency f (MHz) and a width w (mm) of the conductor satisfy following formulae (1) and (2), respectively:
f=Ax+By+C(MHz)xe2x80x83xe2x80x83(1) 
w=Dx+E (mm)xe2x80x83xe2x80x83(2) 
where
y represents a width (mm) of the base body; and
A, B, C, D, and E each represent a constant which is determined in accordance with the thickness a, the length b, and the relative dielectric constant xcex5r or the relative magnetic permeability xcexcr of the base body.
According to the invention, in correspondence with the thickness, length, and relative dielectric constant of the base body and the number of winding of the conductor under predetermined conditions, the resonant frequency and the width of the conductor are each so set as to satisfy the predetermined formula. In this way, a helical antenna having a desired resonant frequency can be designed with ease in accordance with the formulae. Moreover, although the relationship between the width of the helically-configured conductor and the resonant frequency has not been theoretically clarified yet, if the radiating electrode is fabricated from the helically-configured conductor having a width which is so set as to satisfy the formula, the resultant relationship can be such that the resonant frequency is affected little by a variation in the conductor width. Thus, even if there is for example a 5% variation in the conductor width, a variation in the resonant frequency can be reduced to 1% or below with respect to the designed resonant frequency.
According to the invention, it is possible to realize a compact helical antenna having desired antenna characteristics, with which its resonant frequency, conductor width, and base body width can be designed with ease. It is also possible to provide a helical antenna in which, even if a variation occurs in the conductor width in course of manufacture, a variation in the targeted resonant frequency can be suppressed to the level where no problem arises in practical use.
The invention also provides a communication apparatus comprising the helical antenna according to the invention as described above.
Specifically, the invention provides a communication apparatus comprising a helical antenna including a base body made of a dielectric material or a magnetic material, and a helically-configured conductor formed at least either on a top surface of the base body or in an interior thereof,
wherein, in the base body of the helical antenna, a thickness a (mm) is kept in a range of 0.3xe2x89xa6axe2x89xa63 (mm); a length b (mm) is kept in a range of 5xe2x89xa6bxe2x89xa620 (mm); and a relative dielectric constant xcex5r is kept in a range of 3xe2x89xa6xcex5rxe2x89xa630 or a relative magnetic permeability xcexcr is kept in a range of 1xe2x89xa6xcexcrxe2x89xa68, and also a number of winding x (turn) of the conductor is kept in a range of 3xe2x89xa6xxe2x89xa616,
and wherein a resonant frequency f (MHz) and a width w (mm) of the conductor satisfy following formulae (1) and (2), respectively:
f=Ax+By+C(MHz)xe2x80x83xe2x80x83(1) 
w=Dx+E (mm)xe2x80x83xe2x80x83(2) 
where
y represents a width (mm) of the base body; and
A, B, C, D, and E each represent a constant which is determined in accordance with the thickness a, the length b, and the relative dielectric constant xcex5r or the relative magnetic permeability xcexcr of the base body.
According to the invention, even if there is for example a 5% variation in the conductor width of the helical antenna, a variation in the resonant frequency can be reduced to 1% or below with respect to the designed resonant frequency. Thus, it is possible to realize a communication apparatus incorporating a downsized helical antenna that is excellent in antenna characteristics stability.
In the invention, it is preferable that the base body is made of alumina ceramics or forsterite ceramics.
In the invention, it is preferable that the base body is made of tetrafluoroethylene or glass epoxy.
In the invention, it is preferable that the base body is made of YIG (Yttrium Iron Garnet), Nixe2x80x94Zr compound or Nixe2x80x94Coxe2x80x94Fe compound.